The field of the invention relates generally to gas turbine engine fuel systems and more specifically to the optimization of such fuel systems dependent upon the type of fuel being used in the system.
The various jet fuel types are governed by standards developed by the American Society for Testing and Materials (ASTM). These standards set specification constraints for the quality of aviation turbine fuels and describes the fuels found satisfactory for the operation of aircraft and turbine engines. To be utilized for aviation, each jet fuel type must meet or exceed the standards for aviation fuels. Current gas turbine engine and commercial aircraft research and development efforts are focusing on the practicality of using alternative fuels in near-term, mid-term, and far-term aircraft. However, most known alternative fuels do not comply with the ASTM established standards, and many are non-compliant outside of the established specifications by 2-20%. To bring such fuels into compliance, significant processing steps may need to be taken to convert such fuels into the allowable specification range. Such additional processing steps may render the alternative fuels financially impractical.
Current efforts are underway to certify “drop in” jet fuels as alternatives to typical aircraft fuels, such as Jet-A, for example. A “drop in” fuel, i.e. direct replacement, is a fuel that is capable of being blended with, or completely replacing Jet-A fuel without necessitating any substantial modifications to the aircraft or engine. Some known “drop in” fuels, which consist primarily of a blend of kerosene and other synthetic fuels, are currently available for use in existing and near-term aircraft.
However, future mid-term and long-term aircraft development projects propose using a wide variety of bio-jet and synthetic fuels (as blends and in unmixed or “neat” form) for use in ultra-efficient airplane designs. Such fuels are substantially similar in performance to conventional jet fuel, but may have a near-zero sulfur and aromatics content resulting in substantially lower particulate exhaust emissions. In addition, synthetic fuels exhibit excellent low-temperature properties, maintaining a low viscosity at lower ambient temperatures.
As mentioned above, many alternative fuels have properties that do not fit within current ASTM fuel specifications, or that vary within specification limits and have different relationships between properties. Such fuels may be considered “near drop-in” fuels, as their usage would require only minor system adjustments to aircraft systems to allow for safe and efficient use. Some known methods of optimizing fuel systems in response to these differing fuel properties include conducting a major ground and flight test program to obtain approval of each alternative fuel or fuel combination. Additionally, alternative fuels may be heavily processed to attempt to meet all or nearly all fuel specification limits, which can add significant cost to the fuel. Currently, there is no reliable way to quickly determine the type of fuel being used in the aircraft in combination with adjusting aircraft and engine systems to use “near drop-in” fuels. Such a method would add flexibility to aviation fuel specifications, and enable a wider variety of alternative fuels to be utilized in military and commercial aircraft. The above-mentioned flexibility could broaden options for a more environmentally friendly aviation fuel supply, reduce fuel cost, improve fuel quality assurance, and provide a more robust supply if conventional fuel supplies are disrupted.